Adds a private key (as returned by dumpprivkey) to your wallet. Requires a new wallet backup.
Hint: use importmulti to import more than one private key.

Note: This call can take over an hour to complete if rescan is true, during that time, other rpc calls
may report that the imported key exists but related transactions are still missing, leading to temporarily incorrect/bogus balances and unspent outputs until rescan completes.